Objectives: There is little information regarding the reliability of repeat tuberculin skin tests (TSTs) and interferon gamma release assays (IGRAs) in detecting latent tuberculosis infection (LTBI) in people on anti-tumour necrosis factor (TNF) medication.
Methods: We conducted a prospective, observational study of patients referred to the Saskatoon Tuberculosis (TB) Clinic prior to starting anti-TNF medication. A chest x-ray (CXR), 2-step TST and IGRA (QuantiFERON-TB Gold In-Tube Method) were performed at baseline. Those patients with a baseline TST ≥10 mm and/or a positive IGRA were followed with a clinic visit, CXR, TST and IGRA at 3 and 6 months after starting anti-TNF medication.
Results: Of 106 potential patients, 91 consented to participate. Twenty-six patients had a positive (≥ 10 mm) TST or IGRA at baseline; twelve started and stayed on anti-TNF medication through the 6-month follow-up and completed both planned follow-up visits. The baseline mean TST measurement for the 12 participants was 13.9 mm (SD 11.4), increasing to a mean of 16.8 mm (SD 9.3) post-booster. At 3 months post-anti-TNF initiation, there was an overall decrease in TST measurement (mean=10.0 mm; SD 9.3; p=0.013), with measurements <5 mm in 3 of the 12 patients. By the 6-month TST, a response recovery was observed with a mean TST measurement of 14.5 mm (SD 7.7), with 11/12 ≥5 mm. The IGRA was unchanged throughout the study period in all patients. The overall agreement between TST and IGRA was poor (kappa coefficient = 0.180, p=0.020).
Conclusions: We demonstrated a transient but significant decrease in TST response in the first six months of anti-TNF therapy.